1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array substrate for an in-plane switching liquid crystal display (IPS LCD) device and a method of manufacturing the same.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device has upper and lower substrates, which are spaced apart and face each other, and a liquid crystal layer between the upper and lower substrates. Each of the substrates includes an electrode, and the electrodes of each substrate also face each other. The LCD device uses an optical anisotropy of liquid crystal and produces an image by controlling light transmissivity by varying the arrangement of liquid crystal molecules, which are arranged by an electric field.
Because LCD devices have high resolution and can display an excellent moving image, they are used widely. Typically, an LCD device includes thin film transistors and pixel electrodes arranged in a matrix. Such an LCD device is referred to as an active matrix liquid crystal display (AMLCD).
In the LCD device, the liquid crystal layer generally is driven by an electric field that is perpendicular to the upper and lower substrates. The LCD device has a high transmittance and a high aperture ratio. The common electrode of the upper substrate may be grounded so that breakdown of the device due to static electricity may be prevented.
However, the LCD device has a disadvantage of a narrow viewing angle. To overcome the narrow viewing angle, an in-plane switching (IPS) LCD device has been developed. The IPS LCD device implements an electric field that is parallel to the substrates. A detailed explanation of a conventional IPS LCD device and its operation modes will be provided with reference to the following figures.
FIG. 1 is a schematic cross-sectional view of a related art in-plane switching liquid crystal display (IPS LCD) device. As shown in FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 12 is interposed there between. The upper and lower substrates 10 and 20 are referred to as a color filter substrate and an array substrate, respectively. Common and pixel electrodes 36 and 38 are disposed on the lower substrate 20. The common and pixel electrodes 36 and 38 are parallel with each other and spaced apart from each other. Molecules of the liquid crystal layer 12 are aligned by a lateral electric field 21 between the common and pixel electrodes 36 and 38 when voltage is applied to the common and pixel electrodes 36 and 38.
FIGS. 2A and 2B are cross-sectional views illustrating operations of the liquid crystal molecules for IPS mode in “on” and “off” states. FIG. 2A conceptually illustrates “off state” operation modes for a related art IPS LCD device. In the off state, the long axes of the liquid crystal molecules 12 are parallel to the common and pixel electrodes 36 and 38 on the lower substrate 20, and maintains an initial arrangement according to an alignment layer, which is made by a method such as a rubbing.
FIG. 2B conceptually illustrates “on state” operation modes for a related art IPS LCD device. In the “on state”, an in-plane electric field 21 parallel to the upper and lower substrates 10 and 20 is generated between the common and pixel electrodes 36 and 38. The common electrode 36 and pixel electrode 38 are formed together on the lower substrate 20 for this reason. Thus, in an “on state” most of the liquid crystal molecules 12b are aligned such that the long axes thereof are parallel to the substrates 10 and 20 and perpendicular to the common and pixel electrodes 36 and 38, while the liquid crystal molecules 12a over the common and pixel electrodes 36 and 38 maintain an initial arrangement e.g., parallel to the common and pixel electrodes 36 and 38.
As stated above, the IPS LCD device uses the lateral electric field that results from the common and pixel electrodes 36 and 38 being formed on the same substrate, e.g., the lower substrate 20. The IPS LCD device has a wide viewing angle and low color dispersion. Specifically, the viewing angle of the IPS LCD device may be within a range of about 80 to 85 degrees in the directions up, down, right, and left. In addition, the fabricating processes of this IPS LCD device are simpler than other various LCD devices.
FIG. 3 is a plan view of an array substrate for a related art in-plane switching liquid crystal display (IPS LCD) device. As illustrated in FIG. 3, a gate line 32 is formed horizontally in the context of the figure, and a data line 44 extends vertically in the context of the figure. The gate line 32 and the data line 44 cross each other to define a pixel area “P”. A thin film transistor “T” is formed at the crossing of the gate line 32 and the data line 44. The thin film transistor “T” includes a gate electrode 34, a source electrode 46, a drain electrode 48, and an active layer 40. The gate electrode 34 is a part of the gate line 32 and the source electrode 46 is connected to the data line 44.
A common line 37 is formed parallel to the gate line 32. A common electrode 36 is formed in the pixel area “P”. The common electrode 36 includes a plurality of first and second vertical parts 36a and 36b and a horizontal part 36c. The horizontal part 36c overlaps the common line 37 and is connected to the common line 37 through a first contact hole 53. The plurality of first and second vertical parts 36a and 36b extend up and down from the horizontal part 36c, respectively and are parallel to the data line 44. The first and second vertical parts 36a and 36b are spaced apart from the data line 44.
A pixel electrode 38 is also formed in the pixel area “P”. The pixel electrode 38 is composed of a plurality of vertical parts 38a and a horizontal part 38b. The horizontal part of the pixel electrode 38b overlaps the gate line 32 to form a storage capacitor “C”. The vertical parts of the pixel electrode 38a have an alternating arrangement with the first and second vertical parts of the common electrode 36a and 36b. The pixel electrode 38 is connected to the drain electrode 48 through a second contact hole 54.
The common electrode 36 may be made of the same material as the gate line 32 and the pixel electrode 38 may be made of the same material as the data line 44.
Next, FIGS. 4A and 4B are cross-sectional views along the line IVA-IVA and the line IVB-IVB of FIG. 3, respectively. As shown in the figures, a gate electrode 34 and a plurality of common electrodes 36b, which are referred to as the second vertical parts of common electrode in FIG. 3, are formed on a substrate 20. The plurality of common electrodes 36b may be made of the same material as the gate electrode 34. A gate insulator 35 covers the gate electrode 34 and the plurality of common electrodes 36b. An active layer 40 and an ohmic contact layer 41 are subsequently formed on the gate insulator 35. Additionally, source and drain electrodes 46 and 48 are formed on the ohmic contact layer 41. Meanwhile, a data line 44 and which is made of the same material as the source and drain electrodes 46 and 48, is formed on the gate insulator 35 not overlapping the common electrodes 36b. A passivation layer 52 covers the data line 44, source and drain electrodes 46 and 48. The passivation layer 52 has a contact hole 54, which is referred to as a second contact hole in FIG. 3, exposing the drain electrode 48. A plurality of pixel electrodes 38a are formed on the passivation layer 52 alternating with the common electrodes 36b. 
In generally, the gate insulator 35 is made of an inorganic material such as silicon nitride (SiNx) or silicon oxide (SiO2), which has relatively large dielectric constant. When the data line 44 and the common electrodes 36b overlap each other, a parasitic capacitance is formed between the data line 44 and the common electrodes 36b, and vertical crosstalk occurs due to the parasitic capacitance. Therefore, the data line 44 and the common electrodes 36b should be spaced apart at a regular interval, which is noted to as “L” in FIG. 4B, not to form a parasitic capacitance.
However, the interval “L” increases the width of a black matrix 42 on a color filter substrate required to prevent leakage light, and this tends to lower the aperture ratio of the IPS LCD device.
Meanwhile, FIG. 5 illustrates distribution of the light transmittance in the related art IPS LCD device when voltage is applied. As shown in the figure, transmittance of light 60 in a first area “A1” over the common electrode 36b is relatively low and the pixel electrode 38a, while the transmittance of light is relatively high in a second area “A2” between the common electrode 36b and the pixel electrode 38a. This is due to difference in alignment directions of liquid crystal molecules (not shown) over the electrodes 36b and 38a and between the electrodes 36b and 38a under the applied voltage. That is, in the first area “A1”, since the direction of the electric field is perpendicular to the electrodes 36b and 38a, the liquid crystal molecules are arranged perpendicular to the electrodes 36b and 38a. On the other hand, in the second area “A2”, as the direction of the electric field is parallel to the electrodes 36b and 38a, the liquid crystal molecules is arranged parallel to the electrodes 36b and 38a. 
Therefore, as the electrodes occupy more space, the aperture ratio of the IPS LCD device decreases. Besides, since the black matrix should cover the space between the common electrode and the data line, the aperture ratio reduces more.
Though the aperture ratio may increase by narrowing the width of the common electrodes and the pixel electrodes, it is also restricted.
FIG. 6 schematically shows distribution of electric field in the related art IPS LCD device. In FIG. 6, when voltage is applied to the common electrode 36b and the pixel electrode 38a, electric field 54, which is virtually expressed as power lines, is formed between the common electrode 36b and the pixel electrode 38a. The electric field 54 is the weakest in the middle “N” of the area between the common electrode 36b and the pixel electrode 38a because the density of the power lines is the sparsest in the region “N”. The power of electric field gets weak as the interval between the common electrode 36b and the pixel electrode 38a becomes wide. If electric field is weak, the liquid crystal molecules do not aligned correctly. Accordingly, there is a limitation on enlarging the interval between the common electrode 36b and the pixel electrode 38a. 